Multi-layer tunneling device with a graded stoichiometry insulating layer

ABSTRACT

An improved and novel multi-layer thin film device including a graded-stoichiometry insulating layer ( 16 ) and a method of fabricating a multi-layer thin film device including a graded-stoichiometry insulating layer ( 16 ). The device structure includes a substrate ( 12 ), a first electrode ( 14 ), a second electrode ( 18 ), and a graded-stoichiometry insulating, or tunnel-barrier, layer ( 16 ) formed between the first electrode ( 14 ) and the second electrode ( 18 ). The graded-stoichiometry insulating tunnel-barrier layer ( 16 ) includes graded stoichiometry to compensate for thickness profile and thereby produce a uniform tunnel-barrier resistance across the structure ( 10 ). In addition, included is a method of fabricating a multi-layer thin film device ( 10 ) including a graded-stoichiometry insulating tunnel-barrier layer ( 16 ) including the steps of providing ( 40 ) a substrate ( 12 ), depositing ( 44 ) a first electrode ( 14 ) on the substrate ( 12 ), depositing ( 46 ) a metal layer ( 21 ) on a surface of the first electrode ( 14 ), reacting ( 50; 52;  or  54 ) the metal layer ( 21 ) to form a insulating tunnel-barrier layer with uniform tunneling resistance ( 16 ) by using a non-uniform reaction process ( 21 ) and depositing ( 56 ) a second electrode ( 18 ) on the uniform tunneling insulating layer ( 16 ).

FIELD OF THE INVENTION

[0001] The present invention relates to a multi-layer thin film deviceincluding an insulating layer, and more particularly to the inclusion ofa graded-stoichiometry insulating layer.

BACKGROUND OF THE INVENTION

[0002] Typically, a magnetic element, such as a magnetic memory element,includes a multi-layer thin film structure including ferromagneticlayers separated by a non-magnetic layer, hereinafter referred to as aninsulating tunnel barrier, or barrier, layer. Information is stored inthe magnetic layers as directions of magnetization vectors. Magneticvectors in one magnetic layer, for instance, are magnetically fixed orpinned, while the magnetization direction of the other magnetic layer isfree to switch in an applied field between the same and oppositedirections that are called “parallel” and “antiparallel” states,respectively. In response to parallel and antiparallel states, themagnetic memory element represents two different resistances. Theresistance has minimum and maximum values when the magnetization vectorsof the two magnetic layers point in substantially the same and oppositedirections, respectively. Accordingly, a detection of change inresistance allows a device, such as an MRAM device, to provideinformation stored in the magnetic memory element. The differencebetween the minimum and maximum resistance values, divided by theminimum resistance is known as the magnetoresistance ratio (MR).

[0003] Thin film structures of this type, more particularly magnetictunnel junction elements, structurally include very thin layers, some ofwhich are tens of angstroms thick. Of present concern is the fabricationof the insulating tunnel barrier, or barrier, layer formed between theferromagnetic layers. The junction resistance varies approximatelyexponentially with the thickness of the barrier layer. This strongdependence on thickness makes it very difficult to produce devices withnearly the same resistance over typical substrate sizes used inmanufacturing.

[0004] There are currently four common methods for forming insulatingtunnel barrier layers made of aluminum oxide: a) natural oxidation ofaluminum (Al) metal; b) ultraviolet (UV) assisted oxidation of aluminum(Al) metal; c) plasma oxidation of aluminum (Al) metal; and d) reactivesputtering of aluminum oxide. It should be understood that whilealuminum oxide is disclosed as forming the insulative tunnel barrier,other materials which include insulative properties are anticipated bythis disclosure, including, aluminum nitride, tantalum oxide, tantalumnitride, or the like.

[0005] The oldest method of forming an insulating tunnel barrier layeris through natural oxidation. Initially a layer of metal, such asaluminum (Al), is deposited on an electrode which is on some type ofsubstrate. The aluminum is exposed to oxygen or air for a period oftime, typically a number of hours. This exposure to oxygen or air causesthe aluminum to oxidize and form an aluminum oxide layer. Subsequentlythe tunnel junction structure is completed by forming on top of theinsulating layer the remaining electrode, ferromagnetic layers, or thelike, dependent upon the type of device being formed.

[0006] One problem that exists with this type of device is that theinitial depositing of the metal, here aluminum (Al), is non-uniform inthickness, and accordingly resultant differences in tunneling resistanceacross the layer occur. In addition, this method has been found to bevery slow and there is limited flexibility in controlling the resistanceof the formed insulating layer. As a result the resistance tends to bevery low. Natural oxidation assisted by exposure to ultraviolet light isfaster and potentially more controllable.

[0007] An alternative method for forming an insulating tunnel barrierlayer, or insulating layer, is through plasma oxidation. During thisprocess, aluminum is deposited onto a substrate and is subsequentlyoxidized with oxygen plasma, such as from an oxygen glow discharge,similar to neon plasma in a neon light. In general, this type oftechnique is hard to control and does not render a uniform thickness,thus resistance, across the substrate due to the lack of controlexhibited over the oxygen discharge. Alternatively, an oxygen plasmasource is utilized to oxidize the aluminum. This method provides for thecontrol of resistance over a wide range and typically highmagneto-resistance values are achieved. This method is generally similarto the previously described method utilizing a plasma discharge, yetmore control can be exercised.

[0008] Yet another alternative method for forming an insulating tunnelbarrier layer is reactive sputtering. During this process, aluminum isdeposited, utilizing reactive sputtering, onto a substrate surface in anoxygen atmosphere. The aluminum reacts with the oxygen enclosed withinthe vacuum chamber to form aluminum oxide. Typically, results tend toexhibit high resistance and the end product is not uniform in thicknessacross the substrate leading to variations in resistance levels.

[0009] Other, less common, methods of producing insulating layers havebeen reported. A method similar to the plasma oxidation method is theoxygen beam method, deposition of an aluminum layer followed by exposureto an oxygen beam. The beam can be, for example, a low-energy oxygen ionbeam or a low-energy atomic oxygen beam. A technique similar to reactivesputtering is sputter deposition from a compound target. This can beused with or without the addition of extra oxygen into the sputteringambient, with or without an oxygen ion assist beam, or with or withoutan extra oxidation step after deposition.

[0010] Generally, while these four methods have been found to be usefulfor the fabrication of tunnel barrier insulating layers, uniformity ofjunction resistance over large wafer sizes used in semiconductormanufacturing, 150 mm to 300 mm diameter, has not been demonstrated. Theresistance of the MTJ material, usually expressed as the resistance-areaproduct (RA), has been shown to vary exponentially with both the metallayer thickness and oxidation dose for thickness and dose values thatproduce high MR. For example, a variation in aluminum thickness of only5% can result in a variation in RA of over 30%. This strong dependenceon the thickness and oxidation dose makes it very difficult to obtainhigh uniformity of RA. However, by adjusting the aluminum thicknessprofile and oxidation profile to offset each other, a resultant barrierlayer with a small variation in thickness which is compensated for by avariation in the composition of the material provides for a uniform RAover the area of a substrate.

[0011] Accordingly, it is an object of the present invention to providefor a multi-layer thin film structure that includes an insulating layerhaving graded stoichiometry to compensate for the thickness profile andproduce a uniform tunneling resistance across the insulating layer, moreparticularly, across the entire wafer structure as fabricated.

[0012] It is another object of the present invention to provide for agraded-stoichiometry insulating layer and method of fabricating thelayer, for use in multi-layer thin film structures.

[0013] It is yet another purpose of the present invention to provide fora graded-stoichiometry insulating layer and method of fabricating thelayer that includes precise control of the stoichiometry of theresultant layer, thus uniformity of device resistance across thesubstrate area.

[0014] It is another purpose of the present invention to provide for agraded-stoichiometry insulating layer and method of fabricating thelayer that provides for uniformity of the resistance-area product intunnel junction material.

[0015] It is still another purpose of the present invention to providefor a graded-stoichiometry insulating layer and method of fabricatingthe layer that includes the control of the lateral profile of theoxidation process, thus resistance of the resultant insulating tunnelbarrier layer.

[0016] It is still a further purpose of the present invention to providefor a laterally-graded-stoichiometry insulating layer and method offabricating the layer that is amenable to high throughput manufacturing.

SUMMARY OF THE INVENTION

[0017] These needs and others are substantially met through provision ofa multi-layer thin film structure including a graded-stoichiometryinsulating layer and a method of fabricating a multi-layer thin filmstructure including a graded-stoichiometry insulating layer. Thestructure includes a substrate, a first electrode, a second electrode,and a graded-stoichiometry insulating, or barrier, layer formed betweenthe first electrode and the second electrode. The insulating layerincludes laterally graded stoichiometry to compensate for its thicknessprofile and thereby produce uniform tunneling barrier resistance acrossthe structure. In addition, disclosed is a method of fabricating amulti-layer thin film structure including a graded-stoichiometryinsulating layer including the steps of providing a substrate,depositing a first electrode on the substrate, depositing a metal layeron a surface of the first electrode, causing the metal layer to react toform a insulating layer with uniform RA by grading the stoichiometry ofthe metal layer, and depositing a second electrode on the uniforminsulating layer.

BRIEF DESCRIPTION OF THE DRAWINGS

[0018]FIG. 1 illustrates a cross-sectional view of a first embodiment ofa complete multi-layer thin film structure formed according to themethod of the present invention;

[0019]FIG. 2 illustrates an enlarged cross-sectional view of agraded-stoichiometry barrier layer, formed according to the presentinvention;

[0020]FIG. 3 illustrates an enlarged cross-sectional view of agraded-stoichiometry barrier layer, formed according to the presentinvention;

[0021]FIG. 4 illustrates an enlarged cross-sectional view of agraded-stoichiometry barrier layer, formed according to the presentinvention; and

[0022]FIG. 5 illustrates in a flow chart diagram, the method of forminga graded-stoichiometry insulating layer according to the presentinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0023] During the course of this description, like numbers are used toidentify like elements according to the different figures thatillustrate the invention. Accordingly, FIG. 1 illustrates incross-sectional view a first embodiment of a multi-layer thin filmstructure, more particularly a magnetic element, formed according to themethod of the present invention. More specifically, illustrated in FIG.1, is a fully patterned magnetic element structure, generally referenced10. Structure 10 includes a substrate 12, a base electrode multilayerstack 14, an insulating spacer, or barrier layer, 16 including oxidizedaluminum, and a top electrode multilayer stack 18. Base electrodemultilayer stack 14 and top electrode multilayer stack 18 includeferromagnetic layers. Base electrode layers 14 are formed on a metallead, or contact layer, 13, which is formed on a substrate 12. Baseelectrode layers 14 include a first seed layer 20, deposited on metallead 13, a template layer 22, a layer of antiferromagnetic pinningstructure 24, and a fixed ferromagnetic layer 26 formed on and exchangecoupled with the underlying antiferromagnetic pinning structure 24.Ferromagnetic layer 26 is described as fixed, or pinned, in that itsmagnetic moment is prevented from rotation in the presence of an appliedmagnetic field up to a certain strength. Ferromagnetic layer 26 istypically formed of alloys of one or more of the following: nickel (Ni),iron (Fe), and cobalt (Co) and includes a top surface. Top electrodestack 18 includes a free ferromagnetic layer 28 and a protective layer30. The magnetic moment of the free ferromagnetic layer 28 is not fixed,or pinned, by exchange coupling, and is free to rotate in the presenceof a sufficient applied magnetic field. Free ferromagnetic layer 28 istypically formed of alloys of one or more of the following: nickel (Ni),iron (Fe), and cobalt (Co). It should be understood that a reversed, orflipped, structure is anticipated by this disclosure. More particularly,it is anticipated that the magnetic element formed by the methoddisclosed herein can include a top fixed, or pinned layer, and thusdescribed as a resultant top pinned structure.

[0024] Referring now to FIGS. 2-4, illustrated in enlargedcross-sectional views are three embodiments showing the formation ofinsulating barrier layer 16. More particularly, illustrated in eachdrawing is a portion of device 10. It should be noted that allcomponents of FIG. 1 that are similar to components of the FIGS. 2-4,are designated with similar numbers, having a single prime, doubleprime, or triple prime added to indicate the different embodiments.Referring now to FIG. 2, illustrated in enlarged cross-sectional view isa structure, generally similar to portion 11 of FIG. 1, illustrating aportion of a thin film structure, referenced 11′. Similar to thestructure described with regard to FIG. 1, this structure includes afirst electrode 14′, a second electrode 18′ and an insulating barrierlayer 16′. In this first embodiment, shown is insulating barrier layer16′ formed having a central aspect 15′ with a central thickness of d_(c)and two opposed edges, referenced 17′, with edge thicknesses of d_(e)which is lesser than thickness d_(c). As disclosed, in this particularembodiment, barrier layer 16′ is formed by depositing a layer ofaluminum (Al) (not shown) onto first electrode 14′. During thedeposition process, the layer has included a variation in thicknessacross the wafer. As illustrated in FIG. 2, the layer of aluminum has athickness variation whereby the central most aspect 15′ of the wafer isthicker in relation to the outer edges 17′. An axially symmetricthickness profile of this kind is typical of films deposited onto arotating substrate. This difference in thickness, results in a variancein resistance across the wafer, once the layer is oxidized to formresultant oxidized layer 16′.

[0025] Referring now to FIG. 3, illustrated is a portion 11″ of adevice, generally similar to the embodiment described with reference toFIG. 2, except in this particular embodiment a central most aspect 15″of oxidized layer 16″, having a thickness of d_(c), is the thinnest andthe outer edges 17″ of layer 16″, having a thickness d_(e), are thethicker. As previously stated, this difference in thickness, results ina variance in resistance across the wafer once the deposited metal layeris oxidized to form resultant oxidized layer 16″.

[0026] Referring now to FIG. 4, illustrated is a portion 11′″ of adevice, generally similar to the embodiment described with reference toFIGS. 2 and 3, except in this particular embodiment, layer 16′″ isformed having a wedge-shaped formation across the wafer. Moreparticularly, the thinnest aspect of layer 16′″ is formed at one edge 19of the wafer, and the thickest aspect of layer 16′″ is formed at anopposed edge 23. This type of fabrication is typically found whereshutter motion during deposition produces a thickness variation acrossthe wafer from edge 19 to opposed edge 23. As previously stated, thisdifference in thickness, results in a variance in resistance across thewafer, once the deposited metal layer is oxidized to form resultantoxidized layer 16′″.

[0027] As illustrated in FIG. 5, by flow chart diagram, the method offabricating device 10 includes the steps of providing 40 substrate 12,and depositing 42 on an uppermost surface of substrate 12, metal lead13. Metal lead 13 is deposited utilizing standard deposition techniqueswell known in the art. Next, first electrode 14 is formed by depositing44 seed layer 20, template layer 22, antiferromagnetic pinning structurelayer 24, and fixed ferromagnetic layer 26 on an uppermost surface ofmetal lead 13 utilizing standard deposition techniques well known in theart. Next, a layer of metal 21 is deposited 46 on first electrode 14utilizing sputtering techniques well known in the art. Next, it must bedetermined 48 if metal layer 21 is axially symmetric over the waferarea, as illustrated in FIGS. 2 and 3, or non-axially symmetric over thewafer area, as illustrated in FIG. 4. If axial symmetry exists, thenlayer 21 is oxidized utilizing an axially symmetric method, such as byusing an axial symmetric flux profile 50, a rotating wafer technique 52,or a rotating shutter technique 54 to form oxidized insulating layer 16.

[0028] Ion sources typically produce beams with axially symmetriccurrent distributions. During ion beam oxidation, the thickness acrossmetal layer 21 is compensated for by choosing an appropriate holepattern in the grids utilized in an ion beam source. In the instancewhere the thickness of metal layer 21 is greatest in the central mostaspect, as illustrated in FIG. 2, then the grids are chosen to allow fora greater beam current density toward the edges. In this instance, theoxidized film becomes more oxygen rich with increasing radius tocompensate for the thickness variation. The result is a barrier, moreparticularly oxidized layer 16, with a small variation in thicknesswhich is compensated for by a variation in the composition, orstoichiometry, of the oxidized aluminum 16, or other oxidized metal, toprovide for a uniform resistance-area product. It should be understoodthat a variation to compensate for a greater thickness toward the edges17″, as illustrated in FIG. 3, is anticipated by this disclosure. Inthis instance, grid choice would allow for a greater beam current in thecentral most aspect 15″, thereby causing the central aspect 15″ tobecome more oxygen rich than the edges 17″, and thus compensating forthe thickness variation. During ion beam exposure, layer of metal 21,deposited on the first electrode is exposed to a low energy oxygen ionbeam (<200 eV) and oxidized. This oxidation of layer 21 provides for theformation of a thin insulating tunneling layer 16 of aluminum oxide. Itshould be understood that insulating layer 16 can be formed by exposingthe metal to a nitrogen beam, thereby forming, for example, aluminumnitride. During operation, the ion beam voltage and current determinethe oxidation rate. The profile of the ion beam can be adjusted toachieve the desired uniformity of oxidation of metal layer 21. Aspreviously stated, the uniformity of the resultant layer 16 can befine-tuned by adjusting the profile of the beam with the series ofgrids, dependent upon grid design. A larger diameter hole or a higherdensity of holes can be used to increase the beam current density inareas as desired. The grids are formed to provide for compensation inmetal layer 21 thickness variations. For example, the grids can beformed to allow for greater oxidation of layer 21 about the perimeterwhere the thickness is less. This provides for a greater flux of oxygen,thus greater oxidation of this perimeter aspect and uniformity ofresistance across the resultant insulating layer 16.

[0029] An alternative method to address the thickness variation in anaxially symmetric structure is with plasma oxidation. In this instance,the aluminum thickness profile and plasma density profile are adjustedto offset each other. The result is a barrier, or insulating tunnelinglayer, with a small variation in thickness which is compensated for by avariation in the composition of the oxidized aluminum to provide for auniform resistance-area product. During plasma oxidation, the plasmageometry is adjusted to allow for higher plasma density across thethinnest aspect of metal layer 21. The plasma geometry can be adjustedin several ways, including; (a) arrangement of the electrodes or coilsthat excite the plasma, (b) placement of baffles or slits between theplasma and the substrate, or (c) electrodes that direct the ions in theplasma.

[0030] Accordingly, for a device structure similar to that illustratedin FIG. 2, having the thickest aspect in a central portion 15″, theplasma density is adjusted to allow for a higher plasma density aboutthe edges 17″, thus edge heavy oxidation. Where the thickest aspect ofthe metal layer is about the edges 17″, the plasma density is adjustedto allow for a higher plasma density at a central aspect 15″, thuscenter heavy oxidation. This adjustment of the plasma density providesfor a resultant change in the stoichiometry of oxidized layer 16″,without a physical correction in layer thickness.

[0031] Other techniques with axial symmetry, such as UV assistedoxidation with optics that produce an axially symmetric light intensityprofile, also can be used to produce the necessary graded stoichiometry.While undergoing oxidation by any technique, three typical techniquesare used: the flux profile may be axial symmetric with or without theneed for rotating the substrate, the wafer may be rotated with orwithout an axial symmetric source, or a shutter may be rotated betweenthe source and the wafer with or without an axial symmetric source toensure that the average oxidation profile is axially symmetric.

[0032] Finally, where axial symmetry does not exist, more specifically,layer 21 has some thickness gradient that does not have axial symmetry,as illustrated in FIG. 4, metal layer 21 is oxidized with an opposedshutter motion 56 that produces a longer exposure for the thinner areasof layer 21, or a substrate sweeping motion 58 thereby achieving aresultant graded stoichiometry across oxidized layer 16, and uniformresistance. Once proper oxidation of layer 21, with resultant oxidizedlayer 16, a second electrode 18 is formed 60 on oxidized layer 16.

[0033] During fabrication of insulating layer 16, sputtering of a metalmaterial onto first electrode 14 is typically done within a chamber. Thechamber is of the type typically used in the art and is under a vacuumpressure. The vacuum chamber has located within, a sputtering tool fordeposition of the plurality of thin film layers. During fabrication ofdevice 10, substrate 12, having formed thereon first electrode 14 ispositioned within the chamber to allow for the sputtering of a metalfilm 21 upon the surface of substrate 12, more particularly firstelectrode 14. Metal layer 21 is typically formed between 5-15 Å thick.Once the sputtering of metal film 21 is complete, the substrate 12having formed thereon the plurality of thin films, is positioned toallow for the exposing of metal film 21 to achieve oxidation, dependentupon the existence of axial, or non-axial, symmetry. It should beunderstood that while this method of fabricating a thin, uniforminsulating layer 16 is described with reference to the fabrication of amagnetic element 10, it is anticipated that it can be used in allinstances where the fabrication of a thin insulating layer with uniformproperties is desired in a multi-layer thin film structure.

[0034] Thus, an insulating tunneling layer, having a gradedstoichiometry across the layer and thus uniform resistance across thelayer and a method of fabricating the layer is described that providesfor a more manufacturable device and process of making the device, whileremaining cost effective. As disclosed, this technique can be utilizedduring the fabrication of MRAM bits and magnetic field sensors as inhard disk heads. In addition, the technique can be utilized during thefabrication of any multi-layer thin film structure which includes thininsulating layers such as those found in x-ray of EUV mirrors.Accordingly, such instances are intended to be covered by thisdisclosure

What is claimed is:
 1. A multi-layer thin film device comprising: asubstrate having a surface; a first electrode formed on the surface ofthe substrate; a graded-stoichiometry insulating tunnel-barrier layerformed on a surface of the first electrode, the graded-stoichiometryinsulating tunnel-barrier layer including uniform tunneling resistanceacross the layer and a lateral composition gradient to compensate for aninitial material thickness gradient; and a second electrode formed on asurface of the graded-stoichiometry insulating tunnel-barrier layer. 2.A multi-layer thin film device as claimed in claim 1 , wherein thegraded-stoichiometry insulating tunnel-barrier layer is formed of analuminum oxide.
 3. A multi-layer thin film device as claimed in claim 2, wherein the aluminum oxide has a lateral composition gradient tocompensate for an initial aluminum thickness gradient.
 4. A multi-layerthin film device as claimed in claim 3 , wherein thegraded-stoichiometry aluminum oxide insulating tunneling layer includesa uniform tunneling resistance across the layer.
 5. A multi-layer thinfilm device as claimed in claim 1 , wherein the graded-stoichiometryinsulating tunnel-barrier layer is more oxygen rich at opposed edgesthan in the center.
 6. A multi-layer thin film device as claimed inclaim 1 , wherein the graded-stoichiometry insulating tunnel-barrierlayer is more oxygen rich at a central aspect than at opposed edges. 7.A multi-layer thin film device comprising: a substrate having a surface;a metal lead formed on a surface of the substrate; a first electrodeformed on the surface of the metal lead, the first electrode including aferromagnetic layer; a graded-stoichiometry insulating tunnel-barrierlayer formed on a surface of the first electrode, thegraded-stoichiometry insulating tunnel-barrier layer including uniformtunneling resistance across the layer and a lateral composition gradientto compensate for an initial material thickness gradient; and a secondelectrode formed on a surface of the graded-stoichiometry insulatingtunnel-barrier layer, the second electrode including a ferromagneticlayer.
 8. A multi-layer thin film device as claimed in claim 7 , whereinthe graded-stoichiometry insulating tunnel-barrier layer is formed of analuminum oxide.
 9. A multi-layer thin film device as claimed in claim 7, wherein the graded-stoichiometry insulating tunnel-barrier layer isformed of a tantalum oxide.
 10. A multi-layer thin film device asclaimed in claim 7 , wherein the graded-stoichiometry insulatingtunnel-barrier layer is more oxygen rich about opposed edges than at acentral aspect.
 11. A multi-layer thin film device as claimed in claim 7, wherein the graded-stoichiometry insulating tunnel-barrier layer ismore oxygen rich at a central aspect than at opposed edges.
 12. Amulti-layer thin film device as claimed in claim 7 , wherein thegraded-stoichiometry insulating tunnel-barrier layer is more oxygen richon one edge than at an opposed edge.
 13. A method of fabricating amulti-layer thin film structure comprising the steps of: providing asubstrate having a surface; forming a first electrode on the surface ofthe substrate, the first electrode having a top surface and a bottomsurface; forming a metal layer on the top surface of the firstelectrode; reacting the metal layer to form a graded-stoichiometryinsulating tunnel-barrier layer having a top surface and uniformtunneling resistance across the layer; and forming a second electrode onthe top surface of the graded-stoichiometry insulating tunnel-barrierlayer.
 14. A method of fabricating a multi-layer thin film structure asclaimed in claim 13 , wherein the step of forming a metal layer on a topsurface of the first electrode, includes an aluminum material.
 15. Amethod of fabricating a multi-layer thin film structure as claimed inclaim 13 , wherein the step of forming a metal layer on a top surface ofthe first electrode, includes a tantalum material.
 16. A method offabricating a multi-layer thin film structure as claimed in claim 13 ,wherein the step of reacting the metal layer to form agraded-stoichiometry insulating tunnel-barrier layer includes exposureto an ion beam having a non-uniform beam current profile.
 17. A methodof fabricating a multi-layer thin film structure as claimed in claim 13, wherein the step of reacting the metal layer to form agraded-stoichiometry insulating tunnel-barrier layer includes exposureto a plasma having a non-uniform density.
 18. A method of fabricating amulti-layer thin film structure as claimed in claim 13 , wherein thestep of reacting the metal layer to form a graded-stoichiometryinsulating tunnel-barrier layer includes a shutter motion that exposesone area of the layer for a greater period of time than remaining areasof the layer.
 19. A method of fabricating a multi-layer thin filmstructure as claimed in claim 13 , wherein the step of reacting themetal layer to form a graded-stoichiometry insulating tunnel-barrierlayer includes edge heavy reaction.
 20. A method of fabricating amulti-layer thin film structure as claimed in claim 13 , wherein thestep of reacting the metal layer to form a graded-stoichiometryinsulating tunneling layer includes heavy reacting at a central aspect.21. A method of fabricating a multi-layer thin film structure as claimedin claim 13 , wherein the step of reacting the metal layer to form agraded-stoichiometry insulating tunnel-barrier layer includes exposureto an atomic beam having a non-uniform beam flux profile.
 22. A methodof fabricating a multi-layer thin film structure as claimed in claim 13, wherein the step of reacting the metal layer to form agraded-stoichiometry insulating tunnel-barrier layer includes exposureto an ultra-violet light having a non-uniform intensity profile.
 23. Amethod of fabricating a multi-layer thin film structure as claimed inclaim 13 , wherein the step of reacting the metal layer to form agraded-stoichiometry insulating tunnel-barrier layer includes reactingthe metal layer with oxygen to form the insulating tunnel-barrier layer.24. A method of fabricating a multi-layer thin film structure as claimedin claim 13 , wherein the step of reacting the metal layer to form agraded-stoichiometry insulating tunnel-barrier layer includes reactingthe metal layer with nitrogen to form the insulating tunnel-barrierlayer.